I started a thread about this on the ultimate sub-reddit recently. It’s very frustrating in an adult league or lower-level tournament when the other team doesn’t use facts or the rules when making calls. I gave examples of a receiver taking several steps after catching then using that new position to check feet, and a receiver landing on the line and leaving a footprint and refusing to acknowledge this. Shouldn’t getting the call right be important? And often the sideline players have a better perspective than any of the 14 players in that point. They are standing, don’t have to worry about being in position so can focus completely on watching the play, and often are immediately next to the play while an on-field player might be 40 meters away.
I can see that taken to an extreme this will lead to an unpleasant situation for all, and I wouldn’t want the sideline calling fouls or travels or picks or double-teams, but in clear-cut binary calls with no subjectivity of whether it affected the play, I think the sideline (they are playing in the game, after all, and any point is part of the game) could be more involved. Certainly I wouldn’t want to see someone punished for speaking up about a clear bad call.
Also I am not suggesting this for local youth play (though I would like to see a coach being able to tell their own players they were not in or that the disc was down)